This program is concerned with the development and study of new strains of rats and mice; and continued examinations of current strains of aging germfree rats and mice. It is concerned with the development of model animal systems for neoplastic diseases - their applications in pathogeneis and in therapeutic procedures. BIBLIOGRAPHIC REFERENCES: Pollard, M. and P. H. Lucker. Transplantable metastasizing adenocarcinomas in rats. J. Nat. Cancer Inst., 54-643-649, 1975. Truitt, R. L. and M. Pollard. Allogeneic bone marrow chimerism in germfree mice. IV. Therapy of Hodgkin's-like reticulum cell sarcoma in SJL mice. Transplantation, 21:12-16, 1976.